An aircraft propulsion system comprises a nacelle in which a power plant that drives a fan that is mounted on its shaft is arranged in an essentially concentric manner.
The nacelle comprises an inside wall that delimits a pipe with an air intake at the front, whereby a first portion of the incoming air flow, called primary flow, passes through the power plant to participate in the combustion, and whereby the second portion of the air flow, called secondary flow, is driven by the fan and flows into an annular pipe that is delimited by the inside wall of the nacelle and the outside wall of the power plant.
The noise that is emitted by the propulsion system consists of, on the one hand, jet noise, produced outside of the pipes following the mixing of different flows of air and exhaust gases, and, on the other hand, noise that is generated by the inside portions, called internal noise, produced by the fan, the compressors, the turbines and the combustion that propagates inside the pipes.
To limit the impact of noise pollution close to airports, the international standards are increasingly restrictive in terms of sound emissions.
Techniques have been developed to reduce the internal noise, in particular by using, at the walls of pipes, coatings whose purpose is to absorb a portion of the sound energy, in particular by using the principle of Helmholtz resonators. In a known manner, this acoustic coating, also called acoustic panel, comprises—from the outside to the inside—an acoustically resistive structure, an alveolar structure, and a reflective layer.
Layer is defined as one or more layers that may or may not be of the same nature.
The acoustically resistive structure is a porous structure that has a dissipative role, partially transforming the acoustic energy of the sound wave that passes through it into heat. It comprises so-called open zones that can allow acoustic waves to pass and other so-called closed or filled zones that do not allow sound waves to pass but are designed to ensure the mechanical strength of said layer. This acoustically resistive layer is characterized in particular by an open surface ratio that varies essentially based on the engine and components that constitute said layer.
Generally, the acoustically resistive structure comprises at least one porous layer and at least one reinforcement structure.
The porous layer is to make it possible to make the acoustic treatment linear and to trap the acoustic waves in the Helmholtz cells that are formed by the alveolar structure.
According to one embodiment, the porous layer is a metal material, in particular a stainless steel mesh that is known to one skilled in the art.
The advantage of this type of material is that it has a significant mechanical strength even for very small thicknesses, on the order of 1 to 2 tenths of a millimeter, greater than that of a synthetic material.
This significant mechanical strength is necessary because this material that is placed on the surface in direct contact with the aerodynamic flows can be eroded by solid particles such as grains of sand and small rocks, or it can be impacted by pieces of ice or birds that may be sucked in that, with speed, can cause degradations.
According to another advantage, this metal material is an excellent conductor for the swept stroke.
The reinforcement structure comes in the form of a composite or metal plate in which orifices with a more or less large cross-section are made. According to one embodiment, the reinforcement structure comes in the form of a panel with oblong and round perforations. As a variant, the panel could comprise micro-perforations with diameters on the order of 0.05 to 1.2 mm.
A metal reinforcement structure and a metal damping material are preferred because they make it possible to obtain a necessary high mechanical strength, in particular when the acoustically resistive structure is inserted in heavily stressed zones, such as a leading edge of an air intake of a nacelle.
In addition, these metal elements ensure an excellent heat diffusion that improves the effectiveness of the frost treatment that is necessary at the air intake of a nacelle.
To assemble the porous layer and the reinforcement structure, the bonding that makes it possible to obtain a smooth surface, and therefore better aerodynamics, and to not increase the on-board weight too much, unlike other attachment means such as rivets, screws, etc., is used. In addition, the bonding makes it possible to assemble various materials, elements of different thicknesses, and to obtain a better distribution of constraints.
According to one embodiment, thermostable thermoplastic resins are used, such as that of the families of polyetherimides (PEI), polyether ether ketones (PEEK), polyphenylenesulfones (PPS), polyamides (PA), and the polyethylene terephthalate (PET), making it possible to obtain behaviors with prolonged exposures to temperatures of between 300° C. and 400° C.
The patent applications FR-2,826,168 in the name of the applicant describe processes for the production of an acoustically resistive layer.
Prior to the assembly of the elements, the reinforcement structure is perforated or micro-perforated, and then cleaned and prepared so that the adhesive adheres correctly to said reinforcement structure.
Next, an adhesive film of constant thickness, non-adhesive under cold conditions, is arranged between the reinforcement structure that is prepared and the porous layer. The film is preferably cut out along the open zones of the reinforcement structure so as not to glue said zones and to block, facing the open zones, the material meshes used as a porous layer.
According to the requirements, these elements can optionally be shaped or folded.
Next, the different elements are heated so as to activate the adhesive and pressed. After cooling, a strong assembly of the reinforcement structure and the porous layer is obtained. This assembly is all the stronger since the adhesive has a constant and minimal thickness over the entire surface of the reinforcement structure.
Despite all of the care provided during the production of the acoustic damping structure, the result is not optimal for the following reasons:
The two elements to be assembled no longer being planar, the thicknesses of the adhesive can vary by several 100ths to 1 mm, which is reflected by a non-homogeneous nature of the connection between the reinforcement structure and the porous layer, increasing the risk of a significant delamination.
Furthermore, the excess adhesive has a tendency to flow toward the zones that are perforated or micro-perforated on the porous layer, which obstructs the material meshes used as a porous layer and considerably reduces the effectiveness of the acoustic treatment.
Finally, cutting the adhesive film and its positioning relative to the reinforcement structure so as to make the open zones of said film and said reinforcement structure cooperate are all the more difficult to implement since the shape of the elements to be assembled is complex.
Also, the purpose of this invention is to overcome the drawbacks of the prior art by proposing a process for the production of an acoustically resistive structure that makes it possible to improve the adhesion between the elements of said structure and not to alter the acoustic damping characteristics of said structure.